EP1228101B1 - Polyethylene moulding compound with an improved escr-stiffness relation and an improved swelling rate, a method for the production thereof and the use thereof - Google Patents

Abstract

The invention relates to a polyethylene moulding compound having a multimodal molecular weight distribution which has an overall density of >=0.940 g/cm<3 >and an MFI190/5 in the range from 0.01 to 10 dg/min. The moulding compound according to the invention comprises an amount of from 30 to 60% by weight of low-molecular-weight ethylene homopolymer A which has a viscosity number VNA in the range from 40 to 150 cm<3>/g, an amount of from 30 to 65% by weight of high-molecular-weight copolymer B comprising ethylene and a further olefin having from 4 to 10 carbon atoms which has a viscosity number VNB in the range from 150 to 800 cm<3>/g, and an amount of from 1 to 30% by weight of ultrahigh-molecular-weight ethylene homopolymer C which has a viscosity number VNC in the range from 900 to 3000 cm<3>/g.The invention also relates to a method for the production of the moulding compound in a three-step process, and to the use thereof for the production of hollow articles.

Description

The present invention relates to a polyethylene molding compound with multimodal Molar mass distribution and a process for producing this molding composition in The presence of a catalytic system consisting of a Ziegler catalyst and a cocatalyst over a multi-stage from successive liquid phase polymerizations existing reaction sequence and from the molding material by blow molding extrusion manufactured hollow body.

Polyethylene is widely used for the production of molded parts and containers used because it is a material with low weight, which nevertheless a particularly high mechanical strength, high corrosion resistance against moisture and water in combination with atmospheric oxygen and absolutely has reliable long-term durability and because polyethylene is a good one Chemical resistance and especially light for bottles, canisters and fuel tanks can be processed in motor vehicles.

EP-A-603,935 already describes a molding compound based on polyethylene, which has a bimodal molecular weight distribution and can also be used for Production of pipes is suitable.

A raw material with an even broader molecular weight distribution is in the US PS 5,338,589 and is produced using a highly active catalyst, which is known from WO 91/18934 and in which the magnesium alcoholate as gel suspension is used. It was surprisingly found that the Use of this material in molded parts, especially in pipes, a simultaneous Improvement of the opposing usually in semi-crystalline thermoplastics Properties of rigidity and tendency to creep on the one hand and resistance to stress cracking and toughness on the other hand.

The known bimodal products are particularly characterized by good ones Processability with an outstanding stress crack-stiffness relation out. This combination of properties is of particular importance in manufacturing of hollow bodies such as bottles, canisters and fuel containers Plastic motor vehicles. In addition to this combination of properties is for the production of plastic hollow bodies has the highest possible swell rate Plastic melt necessary because of the swell rate in blow mold extrusion is directly responsible for the fact that the wall thickness control, the Training of the weld seam and the weldability during production in the company optimally set.

It is known that plastics with high swell rates with so-called Phillips catalysts are polymerization catalysts based on Chromium compounds, let it produce well. The plastics produced in this way have but an unfavorable stress crack-stiffness relation compared to the known plastics with bimodal molar mass distribution.

A method is known from EP-A-0 797 599 which is carried out in successive steps Gas phase and liquid phase polymerizations even a polyethylene with a trimodal molecular weight distribution provides. This polyethylene is already suitable it is very good for the production of hollow bodies in blow molding extrusion lines however, in its processing behavior still in need of improvement, because of still too low swelling rate of the plastic melt.

The object of the present invention was to develop a polyethylene Molding compound with which an even better one compared to all known materials Ratio of rigidity to stress cracking resistance can be realized and the also has a high swelling rate of their melt, which is used in the production of Hollow bodies using the blow molding extrusion process are not only optimal Wall thickness control allows an excellent at the same time Weld seam formation and wall thickness distribution enabled.

This problem is solved by a molding compound of the type mentioned at the beginning, whose characteristic features can be seen in the fact that they contain 30 to 60% by weight of a low molecular weight ethylene homopolymer A, 65 to 30 wt .-% of a high molecular weight Copolymer B from ethylene and another olefin with 4 to 10 carbon atoms and 1 to 30% by weight of an ultra high molecular weight ethylene homo- or copolymer C contains, where all percentages are based on the total weight of the Molding compound.

The invention also relates to a method for producing this molding compound in cascaded suspension polymerization and hollow body from this molding compound excellent mechanical strength properties.

The polyethylene molding composition according to the invention has a density at a temperature of 23 ° C in the range of ≥ 0.940 g / cm ³ and a wide trimodal molecular weight distribution. The high molecular weight copolymer B contains small amounts of up to 5% by weight of further olefin monomer units with 4 to 10 carbon atoms. Examples of such comonomers are 1-butene, 1-pentene, 1-hexene, 1 octene or 4-methylpentene-1. The ultra-high molecular weight ethylene homo- or copolymer C can optionally also contain an amount of 0 to 10% by weight of one or more of the above-mentioned comonomers.

The molding composition according to the invention also has a melt flow _index according to ISO 1133, expressed as MFI _190/5 , in the range from 0.01 to 10 dg / min and a viscosity number VZ _tot , measured according to ISO / R 1191 in decalin at a temperature of 135 ° C. in the range from 190 to 700 cm ³ / g, preferably from 250 to 500 cm ³ / g.

The trimodality can be used as a measure of the location of the focal points of the three Single molar mass distributions using the viscosity numbers VZ according to ISO / R 1191 of the polymers formed in the successive polymerization stages to be discribed. The following ranges are those in the individual Polymers formed to take into account reaction stages:

The viscosity number VZ ₁ measured on the polymer after the first polymerization stage is identical to the viscosity number VZ _{A of} the low molecular weight polyethylene A and, according to the invention, is in the range from 40 to 180 cm ³ / g.

VZ _B of the higher molecular weight polyethylene B formed in the second polymerization stage can be calculated using the following mathematical formula: VZ B = VZ 2 - w 1 · VZ 1 1 - w 1 where w _{1 stands} for the proportion by weight of the low molecular weight polyethylene formed in the first stage, measured in% by weight, based on the total weight of the polyethylene formed in the first two stages with a bimodal molar mass distribution, and VZ ₂ for the viscosity number which is present in the polymer is measured after the second polymerization stage. The value calculated for VZ _B is normally in the range from 150 to 800 cm ³ / g.

VZ _c for the ultra-high molecular homo or copolymer C formed in the third polymerization stage is calculated using the following mathematical formula: VZ C = VZ 3 - w 2 · VZ 2 1 - w 2 where w _{2 stands} for the weight fraction of the polyethylene with bimodal molar mass distribution formed in the first two stages, measured in% by weight, based on the total weight of the polyethylene with trimodal molar mass distribution formed in all three stages, and VZ ₃ for the viscosity number the polymer is measured after the third polymerization stage and which is identical to the VZ _total mentioned above. According to the invention, the value calculated for VZ _C is in the range from 900 to 3000 cm ³ / g.

The polyethylene is in suspension or by polymerization of the monomers Temperatures in the range of 20 to 120 ° C, a pressure in the range of 2 to 60 bar and in the presence of a highly active Ziegler catalyst obtained from a Transition metal compound and an organoaluminum compound is composed. The polymerization has three stages, i.e. in three in a row switched stages performed, the molecular weight in each case with the aid of metered Hydrogen is regulated.

The required for the cascaded driving style described above Long-term activity of the polymerization catalyst is due to a special developed Ziegler catalyst guaranteed. A measure of the suitability of this The catalyst is its extremely high hydrogen responsiveness and its one long period of 1 to 8 h consistently high activity. Specific examples of such a suitable catalyst are those in EP-A-0 532 551, EP-A-0 068 257 and EP-A-0 401 776 listed reaction products of magnesium alcoholates with transition metal compounds of titanium, zirconium or vanadium and an organometallic compound of a metal of groups I, II or III of Periodic table of the elements.

The polyethylene molding composition according to the invention can in addition to the polyethylene contain other additives. Such additives are, for example, heat stabilizers, Antioxidants, UV absorbers, light stabilizers, metal deactivators, Peroxide-destroying compounds, basic costabilizers in amounts from 0 to 10 % By weight, preferably 0 to 5% by weight, but also fillers, reinforcing agents, Plasticizers, lubricants. Emulsifiers, pigments, optical brighteners, flame retardants, Antistatic agents, propellants or combinations of these in total from 0 to 50% by weight, based on the total weight of the mixture.

The molding composition according to the invention is particularly suitable for the production of Hollow bodies such as fuel canisters, chemical-resistant containers, canisters, barrels and bottles by first adding the polyethylene molding compound in an extruder Temperatures in the range of 200 to 250 ° C plasticized and then through a nozzle pressed into a blow mold and cooled there.

Conventional single-screw extruders can be used for processing into hollow bodies with a smooth feed zone as well as a high-performance extruder with a fine groove Cylinder and effective feed are used. The snails are typically designed as decompression screws with a length of 25 to 30 D (D = diameter). The decompression screws have a discharge zone, in which temperature differences in the melt are compensated for and in the relaxation stresses caused by shear are reduced should.

Example 1

The polymerization of ethylene was in one continuous process in three reactors connected in series. In the first reactor became a Ziegler catalyst, sufficient suspending agent, ethylene and Hydrogen fed. The amount of ethylene and hydrogen became so set that one volume part of hydrogen accounted for nine volume parts of ethylene.

The catalyst was a Ziegler catalyst as used in Example 2 of WO 91/18934 is described there, the catalyst component a with the operation number 2.2, and that together with a cocatalyst from an organometallic Compound of a metal of group I, II or III of the periodic table of the Elements is added.

In the first reactor, the catalyst with the cocatalyst and Triethylamine in a ratio of 1:10 (mol / mol) continuously metered in.

The polymerization in the first reactor was carried out at a temperature of 76 ° C and a pressure of 0.78 MPa over a period of 3.3 h at a Hydrogen content in the gas space from 67 to 68 vol .-% carried out.

The suspension from the first reactor was then transferred to a second reactor in which the amount of hydrogen had been reduced to 5 parts by volume in the gas space and the amount of C ₄ comonomer had been increased to 5 parts by volume. The amount of hydrogen was reduced by means of an intermediate H ₂ relaxation.

The polymerization in the second reactor was at a temperature of 84 ° C and a pressure of 0.5 MPa over a period of 54 minutes.

The suspension from the second reactor was transferred into the third reactor via a further H ₂ intermediate relaxation, with which the amount of hydrogen in the gas space of the third reactor is adjusted to 5 5% by volume.

The polymerization in the third reactor was carried out at a temperature of 47 ° C and a pressure of ≤ 0.23 MPa over a period of 30 min.

The polymer suspension leaving the third reactor was removed after the Suspending agent and drying the granulation fed.

The viscosity numbers and proportions w _A , w _B and w _C of polymer A, B and _C for polymer A, B and C produced for the polyethylene molding composition, together with the corresponding data for the molding compositions prepared according to Examples 2 to 4 below, are listed in Table 1 below specified.

Example 2

Example 1 was reproduced with the following changes:
The polymerization in the first reactor was carried out at a temperature of 82 ° C. and a pressure of 0.89 MPa over a period of 2.6 hours with a hydrogen content of 68% by volume in the gas space of the reactor.

The suspension from the first reactor was then transferred to a second reactor in which the amount of hydrogen had been reduced to 10 parts by volume in the gas space of the reactor and the amount of C ₄ comonomer had been increased to 0.7 parts by volume in the gas space of the reactor. The amount of hydrogen was again reduced by means of an intermediate H ₂ relaxation.

The polymerization in the second reactor was at a temperature of 80 ° C and a pressure of 0.37 MPa over a period of 66 minutes.

The suspension from the second reactor was transferred to the third reactor and the amount of hydrogen in the gas space of the third reactor was set to 0.6% by volume and that of C ₄ comonomer to 0.8% by volume.

The polymerization in the third reactor was carried out at a temperature of 80 ° C and a pressure of 0.15 MPa over a period of 36 minutes.

The viscosity numbers and proportions w _A , w _B and w _C of polymer A, B and C applicable to the polyethylene molding composition prepared according to Example 2, together with the corresponding data of the molding compositions prepared according to the other examples, are given in Table 1 listed later.

Example 3

Example 2 was reproduced with the following changes:
The polymerization in the first reactor was carried out at a temperature of 80 ° C. and a pressure of 0.74 MPa over a period of 2.1 hours with a hydrogen content of 65% by volume in the gas space of the reactor.

The suspension from the first reactor was then transferred to a second reactor in which the amount of hydrogen had been reduced to 4.1 parts by volume in the gas space of the reactor and the amount of C ₄ comonomer had been increased to 1.1 parts by volume in the gas space of the reactor. The amount of hydrogen was again reduced by means of an intermediate H ₂ relaxation.

The polymerization in the second reactor was at a temperature of 80 ° C and a pressure of 0.24 MPa over a period of 54 minutes.

The suspension from the second reactor was transferred to the third reactor and the amount of hydrogen in the gas space of the third reactor was set to 1.1% by volume and that of C ₄ comonomer to 0.8% by volume.

The polymerization in the third reactor was carried out at a temperature of 60 ° C and a pressure of 0.12 MPa over a period of 30 minutes.

The viscosity numbers and proportions w _A , w _B and w _C of polymer A, B and C applicable to the polyethylene molding composition prepared according to Example 3 are given in Table 1 together with the corresponding data for the molding compositions prepared according to the other examples.

Example 4

Example 3 was reproduced with the following changes:
The polymerization in the first reactor was carried out at a temperature of 80 ° C. and a pressure of 0.82 MPa over a period of 2.2 hours with a hydrogen content of 74% by volume in the gas space of the reactor.

The suspension from the first reactor was then transferred to a second reactor in which the amount of hydrogen had been reduced to 4.0 parts by volume in the gas space of the reactor and the amount of C ₄ comonomer had been increased to 1.3 parts by volume in the gas space of the reactor. The amount of hydrogen was again reduced by means of an intermediate H ₂ relaxation.

The polymerization in the second reactor was at a temperature of 80 ° C and a pressure of 0.20 MPa over a period of 54 minutes.

The suspension from the second reactor was transferred to the third reactor and the amount of hydrogen in the gas space of the third reactor was set to 1.0% by volume and that of C ₄ comonomer to 1.0% by volume.

The polymerization in the third reactor was carried out at a temperature of 60 ° C and a pressure of 0.08 MPa over a period of 30 minutes.

The viscosity numbers and proportions w _A , w _B and w _C of polymer A, B and C applicable to the polyethylene molding composition prepared according to Example 2, together with the corresponding data of the molding compositions prepared according to the other examples, are given in Table 1 listed later.

Comparative example (VB):

Example 1 was reproduced, with the difference that after the second Reaction stage the polymerization was stopped.

The polymerization in the first reactor was carried out at a temperature of 84 ° C and a pressure of 0.90 MPa over a period of 4.2 h at one Hydrogen content in the gas space of the reactor of 76 vol .-% carried out.

The suspension from the first reactor was then transferred to a second reactor in which the amount of hydrogen had been reduced to 3.0 parts by volume in the gas space of the reactor and the amount of C ₄ comonomer had been increased to 1.9 parts by volume in the gas space of the reactor. The amount of hydrogen was again reduced by means of an intermediate H ₂ relaxation.

The polymerization in the second reactor was at a temperature of 83 ° C and a pressure of 0.21 MPa over a period of 80 minutes.

This resulted in a polyethylene with a bimodal molar mass distribution, as it corresponds to the prior art according to EP-A 603 935. example 1 2 3 4 VB W _A 0.35 0.45 0.55 0.55 00:52 W _B 0.55 0.45 0.35 0.35 0.48 W _C 0.10 0.10 0.10 0.10 0 VZ ₁ [m ³ / g] 80 80 100 60 55 MFR (2) 3.5 2.3 2.3 2.0 0.7 MFR (3) 1.2 0.7 0.55 0.56 --- MFR / 5 [g / 10 '] 1.07 0.55 0.30 0.36 0.4 MFR / 21.6 [g / 10 '] 17.9 11 9.5 13.8 13.4 FRR 21.6 / 5 17 20 31.6 36.3 33.6 VZ _tot [cm ³ / g] 306 325 392 373 329 Density [g / cm ³ ] 0,954 0,952 0.953 0,954 0,954 BZ 0 ° C [kJ / m ² ] 9.6 10.7 12.6 7.8 6 BKM [N / mm ² ] 1270 1200 1240 1280 1275 SR rheometer [%] 200 151 153 143 91 SRB [h] 3.7 16 54.2 54.1 39

• BKM = Biegekriechmodul, gemessen nach ISO 54852-Z4 in N/mm² als Einminutenwert,
• SRB = Spannungsrissbeständigkeit der erfindungsgemäßen Formmasse. Sie wird nach einer internen Messmethode ermittelt. Diese Labormethode ist von M. Fleißner in Kunststoffe 77 (1987), S. 45 ff, beschrieben. Diese Publikation zeigt, dass zwischen der Bestimmung des langsamen Risswachstums im Zeitstandversuch an rundum gekerbten Probestäben und dem spröden Ast der Zeitstandsinnendruckprüfung nach ISO 1167 ein Zusammenhang besteht. Eine Verkürzung der Zeit bis zum Versagen wird durch die Verkürzung der Rissinitiierungszeit durch die Kerbe (1,6 mm/Rasierklinge) in Ethylenglykol als spannungsrissfördemdem Medium bei einer Temperatur von 80 °C und einer Zugspannung von 3,5 MPa erreicht. Die Probenherstellung erfolgt, indem drei Probekörper mit den Abmessungen 10 x 10 x 90 mm aus einer 10 mm dicken Pressplatte heraus gesägt werden. Die Probekörper werden rundum mit einer Rasierklinge in einer eigens dafür angefertigten Kerbvorrichtung (dargestellt in Abbildung 5 in der Publikation von Fleißner) in der Mitte gekerbt. Die Kerbtiefe beträgt 1,6 mm.
• BZ = Bruchzähigkeit der erfindungsgemäßen Formmasse. Sie wird ebenfalls nach einer internen Messmethode an Probestäben mit den Abmessungen 10 x 10 x 80 mm, die aus einer 10 mm dicken Pressplatte herausgesägt wurden, bestimmt. In der bereits erwähnten Kerbvorrichtung werden sechs dieser Probestäbe mit der Rasierklinge in der Mitte gekerbt. Die Kerbtiefe beträgt 1,6 mm. Die Durchführung der Messung entspricht weitgehend der Charpy-Messprozedur nach ISO 179 bei veränderten Probekörpem und veränderter Schlaggeometrie (Widerlagerabstand). Alle Probekörper werden über eine Zeitdauer von 2 bis 3 h auf die Messtemperatur von 0 °C temperiert. Man legt dann einen Probekörper zügig auf das Widerlager eines Pendelschlagwerks gemäß ISO 179. Der Widerlagerabstand beträgt 60 mm. Der Fall des 2 J Hammers wird ausgelöst, wobei der Fallwinkel auf 160 °, die Pendellänge auf 225 mm und die Auftreffgeschwindigkeit auf 2,93 m/sec eingestellt wird. Zur Auswertung der Messung wird der Quotient aus verbrauchter Schlagenergie und Anfangsquerschnittfläche an der Kerbe a_FM in mJ/mm² berechnet. Dabei können nur Werte bei vollständigem Bruch und Schamierbruch als Grundlage für einen gemeinsamen Mittelwert dienen (siehe ISO 179).
• SR = Schwellrate, gemessen in einem Hochdruckkapillarrheometer bei einer Scherrate von 1440 1/s in einer 2/2 Rundlochdüse mit einem konischen Einlauf (Winkel = 15 °) bei einer Temperatur von 190 °C.

The abbreviations of the physical properties in Table 1 have the following meaning:

• BKM = bending creep modulus, measured according to ISO 54852-Z4 in N / mm ² as a one-minute value,
• SRB = stress crack resistance of the molding composition according to the invention. It is determined using an internal measurement method. This laboratory method is described by M. Fleißner in Kunststoffe 77 (1987), p. 45 ff. This publication shows that there is a connection between the slow crack growth in the creep test on all-round notched specimen bars and the brittle branch of the creep pressure test according to ISO 1167. A reduction in the time to failure is achieved by shortening the crack initiation time through the notch (1.6 mm / razor blade) in ethylene glycol as a stress-crack-promoting medium at a temperature of 80 ° C and a tensile stress of 3.5 MPa. The sample is produced by sawing three test specimens with the dimensions 10 x 10 x 90 mm out of a 10 mm thick press plate. The test specimens are notched in the middle with a razor blade in a specially designed notching device (shown in Figure 5 in the publication by Fleißner). The notch depth is 1.6 mm.
• BZ = fracture toughness of the molding composition according to the invention. It is also determined using an internal measurement method on test bars with the dimensions 10 x 10 x 80 mm, which were sawn out of a 10 mm thick press plate. In the notching device already mentioned, six of these test bars are notched with the razor blade in the middle. The notch depth is 1.6 mm. Carrying out the measurement largely corresponds to the Charpy measurement procedure in accordance with ISO 179 with modified test specimens and modified impact geometry (abutment spacing). All test specimens are tempered to the measuring temperature of 0 ° C over a period of 2 to 3 h. A test specimen is then quickly placed on the abutment of a pendulum impact tester in accordance with ISO 179. The abutment distance is 60 mm. The fall of the 2 J hammer is triggered, whereby the fall angle is set to 160 °, the pendulum length to 225 mm and the impact speed to 2.93 m / sec. To evaluate the measurement, the quotient of the impact energy used and the initial cross-sectional area at the notch a _{FM is} calculated in mJ / mm ² . Only values with complete break and hinge break can serve as the basis for a common mean (see ISO 179).
• SR = swell rate, measured in a high pressure capillary rheometer at a shear rate of 1440 1 / s in a 2/2 round hole nozzle with a conical inlet (angle = 15 °) at a temperature of 190 ° C.

The measured values clearly show that the molding composition according to the invention consistently led to better strength properties and was also easier to process during manufacture. For the hollow body test, 3 500 ml round bottles were produced on Bekum BAE under the following conditions and with the following result: example 1 2 3 4 VB 1 basic gap 150 150 150 150 200 Speed [rpm] 17.7 17.7 17.7 17.7 - Blowing time [sec] 15 14.0 12.0 11.2 12 Weight [g] 41.4 37.4 36.0 35.6 38 Wall thickness [mm] 1.2 1.00 0.90 0.85 - Weld thickness [mm] 2.0 2.0 1.0 1.0 (0.5) (V-notch) Melt fracture 2 2 2 2 2 Specks 2 2 2 3 1 comment frosted frosted frosted frosted frosted

It can be seen that the molding composition according to the comparative example is too thin Weld seam forms, which also has a V-notch, which is a weak point represents that can burst under pressure.

Claims (4)

1. A polyethylene molding composition with multimodal molar mass distribution, which has an overall density of ≥ 0.940 g/cm³ and has an MFI_190/5 in the range from 0.01 to 10 dg/min, which comprises from 30 to 60% by weight of low-molecular-weight ethylene homopolymer A having a viscosity number VN_A in the range from 40 to 150 cm³/g, from 30 to 65% by weight of high-molecular-weight copolymer B made from ethylene and from another olefin having from 4 to 10 carbon atoms and having a viscosity number VN_B in the range from 150 to 800 cm³/g, and from 1 to 30% by weight of ultrahigh-molecular-weight ethylene homo- or copolymer C having a viscosity number VN_C in the range from 900 to 3 000 cm³/g.
2. The polyethylene molding composition as claimed in claim 1, which has excellent capability for processing to give blow moldings, expressed in terms of a swell index in the range from 100 to 300%.
3. A process for preparing a polyethylene molding composition as claimed in claim 1, by carrying out the polymerization of the monomers in suspension at temperatures in the range from 20 to 120°C, at a pressure in the range from 2 to 60 bar and in the presence of a high-activity Ziegler catalyst composed of a transition metal compound and of an organoaluminum compound, which comprises conducting the polymerization in three stages, the molar mass of the polyethylene prepared in each stage being regulated with the aid of hydrogen.
4. The use of a polyethylene molding composition as claimed in claim 1 for producing blow moldings, such as fuel tanks, canisters, drums, or bottles, where the polyethylene molding composition is first plastified in an extruder at temperatures in the range from 200 to 250°C and is then extruded through a die into a blow mold, where it is cooled.